1. Field of the Invention
The present invention generally relates to heaters, fixing units and image forming apparatuses, and more particularly to a heater that is provided with a capacitor and is used to heat various materials or apparatuses, for example, a fixing unit that uses such a heater, and an image forming apparatus, such as a copying machine, a printer and a facsimile machine, that uses such a fixing unit and employs an electrophotography technique.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, an image is formed on a recording medium that may be plain paper, OHP or the like. From the point of view of the high speed, image quality and cost of the image formation, the electrophotography technique is popularly employed by the image forming apparatus. According to the electrophotography technique, a toner image is formed on the recording medium, and the toner image is fixed on the recording medium by applying heat and pressure. From the point of view of safety and the like, a heat roller type fixing method which uses a heat roller is most popularly employed at present. The heat roller type fixing method forms a mutual pressing part called a nip part where a heat roller and a confronting pressure roller press against each other. The heat roller generates heat from a heating member such as a halogen heater. The recording medium bearing the toner image transferred thereon passes through this nip part, and thus, the toner image is fixed on the recording medium by being applied with the heat and the pressure.
Recently, environmental problems have become increasingly important, and proposals have been made to reduce the power (or energy) consumption of the image forming apparatuses such as copying machines and printers. When reducing the power consumption of the image forming apparatus, it is important to reduce the power consumption of the fixing unit which fixes the toner image on the recording medium. When reducing the power consumption of the fixing unit in a standby state of the image forming apparatus, a popularly employed method maintains a temperature of the heat roller to a constant temperature that is slightly lower than a fixing temperature in the standby state, and immediately raises the temperature of the heat roller to a usable temperature in an operating state where the image forming apparatus is used, so that a user (or operator) does not need to wait for the temperature of the heat roller to rise to the usable temperature. But according to this popularly employed method, a certain power must be supplied to the fixing unit even when the fixing unit is not in use, and surplus power is consumed thereby. It is said that this power consumption in the standby state amounts to approximately 70% to approximately 80% of the total power consumption of devices, units and the like forming the image forming apparatus.
Accordingly, there are demands to reduce the power consumption of the image forming apparatus in the standby state so as to reduce the overall power consumption of the image forming apparatus, and to ultimately make the power consumption zero when the image forming apparatus is not in use. However, if the power consumption of the image forming apparatus in the standby state is set to zero, it will take a long time on the order of several minutes to several tens of minutes to raise the temperature of the heat roller of the fixing unit to the usable temperature of approximately 180° C., because the heat roller is a metal roller mainly made of iron, aluminum or the like having a large heat capacity. Such a long waiting time required until the temperature of the heat roller reaches the usable temperature will make the image forming apparatus inconvenient to use for the user. Hence, there are demands to minimize the power consumption but also enable the temperature of the heat roller to be quickly raised to the usable temperature.
In order to reduce the time required to raise the temperature of the heat roller, it is evident that the input energy per unit time, that is, the rated power, should be set large. In some high-speed image forming apparatuses designed to realize a high printing speed, the power supply voltage is actually set to 200 V. However, in a general office environment in Japan, for example, the commercial power supply is 100 V and 15 A. For this reason, in order for the image forming apparatus to cope with the 200 V power supply, special engineering work needs to be made with respect to power supply related equipments at the setup location of the image forming apparatus. Consequently, the use of the 200 V power supply is not a generally applicable solution for reducing the time required to raise the temperature of the heat roller.
In other words, as long as the commercial power supply of 100 V and 15 A is used, it is difficult to raise the temperature of the heat roller within a short time since a maximum input energy is determined by the power supply. For example, a Japanese Laid-Open Patent Application No. 10-10913 proposes delaying a temperature decrease in the fixing unit during the standby state of the fixing unit by supplying to the heat roller a voltage which is a predetermined level lower than a voltage supplied to the heat roller during the normal operating state of the fixing unit. Further, a Japanese Laid-Open Patent Application No. 10-282821, for example, proposes charging a secondary battery which forms an auxiliary power supply during the standby state of the fixing unit, so that the time required to raise the temperature of the heat roller can be reduced by supplying the power from a main power supply unit, the secondary battery and a primary battery to the heat roller when the normal operating state of the fixing unit is started.
However, the method proposed in the Japanese Laid-Open Patent Application No. 10-10913 must supply to the heat roller the voltage which is the predetermined level lower than the voltage supplied to the heat roller during the normal operating state of the fixing unit, even during the standby state of the fixing unit. Consequently, the power consumption cannot be reduced sufficiently. In addition, making the maximum power supply at the time of starting the normal operating state of the fixing unit higher than the power supplied from the main power supply unit is not the main aim of this proposed method.
On the other hand, according to the method proposed in the Japanese Laid-Open Patent Application No. 10-282821, the power from the main power supply unit, the secondary battery and the primary battery is supplied to the heat roller when the normal operating state of the fixing unit is started. Generally, a lead battery, a nickel-cadmium batter or a nickel-hydrogen battery is used as the secondary battery. The capacity of such secondary batteries deteriorates when repeatedly charged and discharged, and for such secondary batteries, the larger the discharge current the shorter serviceable life. The capacity of such secondary batteries also deteriorate due to memory effect. In general, even a long-life secondary battery, which is designed to have a long serviceable life even when the discharge current is large, will die when the charging and discharging is repeated approximately 500 times to approximately 1000 times. This means that, if the charging and discharging of the secondary battery is repeated 20 times per day, the serviceable life of the secondary battery will expire in approximately 1 month. As a result, it becomes necessary to frequently replace the secondary battery, thereby requiring a troublesome operation of replacing the secondary battery, and also increasing the running cost of the image forming apparatus due to the frequently replaced secondary battery. Furthermore, in the case of the lead battery, it is unsuited for use in office equipments since the lead battery uses sulfuric acid solution as the electrolyte.
In addition, the load on a heater circuit that is built into the heat roller increases, and the making current flows to a peripheral circuit to generate noise, due to a sudden current change, input power and the like when the supply of the large power is started and stopped. For this reason, it is preferably not to frequently turn ON and turn OFF the power supply from the auxiliary power supply having a large capacity. Moreover, when the power from the auxiliary power supply having the large capacity is supplied at once, the excessive supply of power may cause an excessive temperature rise of the heater circuit.
For example, a Japanese Laid-Open Patent Application No. 2002-184554 proposes a method of suppressing the problems described above, that is, increasing the effect of reducing the power consumption, reducing the noise caused by the making current and the sudden current change when the large power is supplied, reducing the time required to raise the temperature of the heat roller, and preventing an excessive temperature rise of the heat roller. This proposed method uses a chargeable and dischargeable capacitor for the auxiliary power supply unit. A charger charges the capacitor of the auxiliary power supply unit by the power supplied from a main power supply unit, a switching unit switches between the charging of the auxiliary power supply unit and the power supply from the auxiliary power supply unit with respect to an auxiliary heater element, and an amount of power supplied from the auxiliary power supply unit to the auxiliary heater element is adjusted. Basic functions of the capacitor include supplying the power from the capacitor to the auxiliary heater element to generate heat therefrom, so that the generated heat can be used to shorten the time required to raise the temperature of the heat roller to the predetermined temperature, and to prevent a fixing temperature from decreasing when the recording medium passes through the fixing unit.
Even in the case of the image forming apparatus that uses the capacitor as the secondary battery to prevent the temperature of the heat roller from decreasing, it would be preferable to refrain from using the capacitor as much as possible when the serviceable life, the reduction of the charging time and the like of the capacitor are taken into consideration, so as to improve the utilization efficiency of the capacitor, particularly if the temperature decrease in the fixing unit will be small or zero without using the capacitor. However, no method has conventionally been proposed to judge if the temperature decrease in the fixing unit will be small or zero even when the capacitor is not used.